Mexico's foreign relations secretary says US President Donald Trump's plan to build a border wall is an "unfriendly, hostile" act.
Foreign Secretary Luis Videgaray said on Tuesday he doesn't think such a wall will accomplish anything, calling it "a bad idea".
Mexico has repeatedly said it will not pay for the wall, despite Trump's repeated assertion he will get the US neighbour to do so.
Videgaray repeated in a meeting with legislators that Mexico "won't pay a cent" for the wall.
Trump had requested Congress provide US funds to begin the wall, but signalled on Monday that he would not insist on it, saying he might be willing to wait until September for the funding.
With the US government set to exhaust its current funding at midnight Friday, the White House is under the gun to craft a compromise that would keep the government running while not appearing to cave on Trump's demand that Congress fund a wall on the border with Mexico.
"Hopefully, we'll reach an agreement some time in the next couple of days," Senate Majority Leader Mitch McConnell said, but he did not rule out passing a short-term extension that gives lawmakers time to craft a final deal.
While lawmaker said the funding legislation would steer clear of including funding for a border wall, it is expected to cover broader border security measures, such as the use of drones or anti-tunnelling technology.
But a deal remained far from certain. Among the sticking points is the fate of cost-sharing arrangements that help reduce health insurance costs for low-income Americans buying plans through the Affordable Care Act, the reform implemented by Trump's predecessor Barack Obama.
Trump had been on a collision course with lawmakers over the wall, demanding that Congress include funding to start construction in its new spending bill.
Democrats blasted the approach, warning that Republicans risked being blamed for shuttering the government barely three months into Trump's administration.
"There aren't enough Republican votes to put the wall in" to the spending bill, said veteran Senator Patrick Leahy, top Democrat on the Appropriations Committee.
With jitters rippling through Washington, Trump apparently agreed to soften his demand days before the deadline.
He told a group of conservative journalists he was open to delaying funding for the wall's construction until September, a White House official said, according to The Washington Post.
By Tuesday, the administration was acknowledging Trump's plan could be pushed back.
"Building that wall and having it funded remains an important priority to him," advisor Kellyanne Conway told Fox News.
"But we also know that that can happen later this year and into next year."
With shutdown fever appearing to break in Washington, top Senate Democrat Chuck Schumer said he was "optimistic" about reaching a timely deal, provided Republicans do not include controversial other issues such as defunding Planned Parenthood clinics.
"We're pleased he's backing off," Schumer said of Trump's funding demand.
Instead of a wall, lawmakers are expressing support for a broader set of border security measures, including using drones and other high technology.
"We're not opposed to border security," Schumer said.
"We are opposed to the wall. And we're not going to allow any dollars in any kind of border security appropriation to be used for a wall. Period."
Building the wall was Trump's signature campaign promise, but Democrats note that Trump pledged Mexico would pay for it.
Even as the White House appeared to soften its rigid posture, Trump sent mixed messages.
"Don't let the fake media tell you that I have changed my position on the WALL. It will get built and help stop drugs, human trafficking etc," he said Tuesday on Twitter.
The president then insisted the wall would be built "soon," but offered no time frame other than within his four-year term.
"The wall's going to get built, folks," Trump told reporters.
Congress, returning from a two-week break, now has three days to strike a deal.
Lawmakers have warned that a stopgap measure, known as a continuing resolution, may be needed to give Congress breathing room to finalize the spending details.
"This is active negotiations," Republican Senator David Purdue said.
With the 100-day mark landing on Saturday and few legislative victories under his belt, Trump is pressing to fulfill another campaign pledge.
As a candidate, he repeatedly vowed to reform the tax code and slash the corporate tax rate from 35 percent to 15 percent.
An announcement on such reforms is due Wednesday.
Conservatives have sought a reduction in tax rates for years, but they are also adamant about reducing the debt. Some Republicans appeared hesitant to embrace such slashes to corporate tax rates.
"Fifteen percent might be too low," House Republican Leonard Lance told CNN.
Democrats have expressed little appetite for dramatically reducing taxes, especially for big business.
If there are no significant tax breaks for small business or working-class Americans, "that would certainly for me be a non-starter," Democratic Senator Debbie Stabenow said.